Optimize the number of replicas required in Quorum read/write

Details

Description

Currently, for C* read/write requests with quorum consistent level, number of replicas required for quorum write is W=N/2+1, and number of replicas required for quorum read is R=N/2+1 as well.

It works fine in odd number of replicas case, which R + W = N + 1, but in even number of replicas case, like RF=4, 6, 8, the R+W = N + 2, which means we are having two overlapping nodes in read/write requests, which is not necessary. It can not provide strong consistency, but will hurts P99 read latency a lot (2X in our production cluster).

In a lot of other database, like Amazon Aurora, they use W = N/2 + 1 and R = N/2 for quorum requests, which will provide enough strong consistency, but talk to one less replica in read path. "We use a quorum model with 6 votes (V = 6), a write quorum of 4/6 (Vw = 4), and a read quorum of 3/6 (Vr = 3)."

I propose we do the same optimization, change read quorum to talk to N/2 replicas, which should reduce the read latency for quorum read in general.